Question For Billfitzmaurice

I would have sent this as an email or a PM but I thought maybe someone else might have a similar question after reading a prievious thread so.........Hey Bill, besides "not looking right", is there anything negative about my set up? I know that I may loose some projection due to the 15 not facing the audience, but that's what PA systems are for. The 2x10 does project outward so I know where those waves are headed, but I'd like to know what happens to the sound waves from the 15. It sound so good to me, but why? Can you take the time to explain this to me or suggest a resource on speaker design and sound theory. Any input would be appreciated.

Simply put, low frequencies go around corners, high frequencies don't. Ideally you'd use a crossover or biamp setup to send only lows (100 Hz and below) to the 15, the rest to the tens. Below 100 Hz soundwaves are basically omnidirectional, so where the speaker is aiming doesn't matter. The highs coming from the 15, though, are wasted, as is the power required to produce them, which is why a crossover or bi-amping would improve the situation. Since the tens don't work well much below 100 Hz you're also wasting power and creating distortion sending them lows that they can't reproduce anyway. You'll never find a hi-fi speaker costing more than $30 that doesn't use some type of crossover to properly allocate frequencies to the appropriate drivers; the fact that you almost never see crossovers used in musical instrument speakers is one more example of how primitive most of them really are.

Long wavelength low frequencies don't radiate into free space from a speaker particularly efficiently. In fact, the average efficiency of a direct radiating driver loudspeaker (one that directs its cone directly to the air) is only about 5% of acoustic power out compared to electrical power in, and the lower in frequency you go the worse the conversion rate is. By in effect converting your direct radiating 15 into one that is assisted by what is known as 'boundary loading' you are actually increasing the efficiency of the speaker in the lowermost frequencies, and that makes them louder, by about 3dB, which translates to a doubling of efficiency. If you were to place the 15 close to a rear wall as well you'd gain another 3dB of sensitivity, and a room corner placement would give another 3dB on top of that.

well, would a Tuba 30 with my 2x10 and a crossover in the chain be possible? I haven't seen a crossover for instrument amps but the concept makes complete sense, (car audio). can you tell me how to make or where to buy a crossover that would work with an amp and cab rig?

A Tuba 30 is overkill big time for backline; Tuba 24 is more than enough. Crossovers aren't difficult, and for your needs aren't expensive either if you buy your own components rather than a finished unit. The Tuba 24 project includes instructions how to wire it with a lowpass filter and how to wire a highpass filter to feed a midbass cab used along with it.

Have you ever seen those passive, line level high pass filters called "F-Mods"? I have used these in RCA plug format but have never seen anything like this in phone plug format?

Can the Auto Tuba be configured for backline bass guitar? I was thinking of something small to support a single 10" combo. It runs 330 watts at 4 ohms so you figure I would have 165 RMS to work with? The output is full range, however.

If you don't want to biamp then you have to use passive(speaker level) components for a crossover. AutoTuba will work for backline bass to moderate volumes, so along with a single 10 midbass should be fine. For a crossover at about 100 Hz you'd run a 10mH inductor in series with the sub driver and a 200mfd capacitor in series with the midbass.

Long wavelength low frequencies don't radiate into free space from a speaker particularly efficiently. In fact, the average efficiency of a direct radiating driver loudspeaker (one that directs its cone directly to the air) is only about 5% of acoustic power out compared to electrical power in, and the lower in frequency you go the worse the conversion rate is. By in effect converting your direct radiating 15 into one that is assisted by what is known as 'boundary loading' you are actually increasing the efficiency of the speaker in the lowermost frequencies, and that makes them louder, by about 3dB, which translates to a doubling of efficiency. If you were to place the 15 close to a rear wall as well you'd gain another 3dB of sensitivity, and a room corner placement would give another 3dB on top of that.

Click to expand...

Good explaination. This is generally why you home Hi-fi sub rarely has the speaker front loaded, and why a lot of people swear by bandpass and folded subs.

But at the end of the day, the lack of crossovers etc on modern instrument speakers isn't so much a case of primitive design, rather a case of trying to achieve a different sound. My Hi-Fi speakers sound very nice with recorded music coming through them but I don't particularly like the way myu bass guitar sounds through them. Conversely, recorded music sounds pretty ordinary through some of my bass cabs. It's all about the preferences and requirements of the end user. The beauty of building cabs for yourself is you can chose which traditions you want to maintain and which you want to break. It's simply a matter of knowing what you want, then setting out to get it.

the lack of crossovers etc on modern instrument speakers isn't so much a case of primitive design, rather a case of trying to achieve a different sound.

Click to expand...

Actually there are two reasons for the lack of crossovers in bass speakers. One is cost; passive crossover components for low frequencies are expensive, and manufacturers hate spending more money than they absolutely have to.

The other is that crossovers are used to differentiate the signals being sent to drivers that don't work over the same frequency range, while musical instrument driver combinations tend to be tossed together willy-nilly rather than logically. The overlap in the frequency response of a fifteen and a ten, for instance, is so broad that you never see that combination used anywhere except in musical instrument speakers, where sadly the level of engineering remains pretty much unchanged in 30 years.

The difference between the sound of your bass directly to hi-fi speakers versus your regular speakers is one of intent. Hi-fi speakers are designed to reproduce music, usually from a recorded source, ideally without adding any coloration of their own. Musical instrument speakers are designed to create music, and being part and parcel of the instrument system coloration is not only acceptable but desireable.